Here we describe a method for the engineered regulation of protein kinases in living cells, the design and application of RapR (rapamycin regulated) kinases. The RapR kinase method enables activation of kinases with high specificity and precise temporal control. Insertion of an engineered allosteric switch, the iFKBP domain, at a structurally conserved position within the kinase catalytic domain makes the modified kinase inactive. Treatment with rapamycin or its non-immunosuppresive analogs triggers interaction with a small FKBP-rapamycin-binding domain (FRB), restoring the activity of the kinase. The reagents used in this method are genetically encoded or membrane permeable, enabling ready application in many systems. Based on the structural similarity of kinase catalytic domains, this method is likely applicable to a wide variety of kinases. Successful regulation has already been demonstrated for three kinases representing both tyrosine and serine/threonine kinase families (p38, FAK, Src). Procedures for designing and testing RapR kinases are discussed.
kinase; allosteric; activation; phosphorylation
The immunosuppressant and anticancer drug rapamycin works by inducing inhibitory protein complexes with the kinase mTOR, an important regulator of growth and proliferation. The obligatory accessory partner of rapamycin is believed to be FK506-binding protein 12 (FKBP12). Here we show that rapamycin complexes of larger FKBP family members can tightly bind to mTOR and potently inhibit its kinase activity. Cocrystal structures with FKBP51 and FKBP52 reveal the modified molecular binding mode of these alternative ternary complexes in detail. In cellular model systems, FKBP12 can be functionally replaced by larger FKBPs. When the rapamycin dosage is limiting, mTOR inhibition of S6K phosphorylation can be enhanced by FKBP51 overexpression in mammalian cells, whereas FKBP12 is dispensable. FKBP51 could also enable the rapamycin-induced hyperphosphorylation of Akt, which depended on higher FKBP levels than rapamycin-induced inhibition of S6K phosphorylation. These insights provide a mechanistic rationale for preferential mTOR inhibition in specific cell or tissue types by engaging specific FKBP homologs.
Cre recombinase is extensively used to engineer the genome of experimental animals. However, its usefulness is still limited by the lack of an efficient temporal control over its activity. We have recently developed a conceptually new approach to regulate Cre recombinase, that we have called Dimerizable Cre or DiCre. It is based on splitting Cre into two inactive moieties and fusing them to FKBP12 (FK506-binding protein) and FRB (binding domain of the FKBP12-rapamycin associated protein), respectively. These latter can be efficiently hetero-dimerized by rapamycin, leading to the reinstatement of Cre activity. We have been able to show, using in vitro approaches, that this ligand-induced dimerization is an efficient way to regulate Cre activity, and presents a low background activity together with a high efficiency of recombination following dimerization. To test the in vivo performance of this system, we have, in the present work, knocked-in DiCre into the Rosa26 locus of mice. To evaluate the performance of the DiCre system, mice have been mated with indicator mice (Z/EG or R26R) and Cre-induced recombination was examined following activation of DiCre by rapamycin during embryonic development or after birth of progenies. No recombination could be observed in the absence of treatment of the animals, indicating a lack of background activity of DiCre in the absence of rapamycin. Postnatal rapamycin treatment (one to five daily injection, 10 mg/kg i.p) induced recombination in a number of different tissues of progenies such as liver, heart, kidney, muscle, etc. On the other hand, recombination was at a very low level following in utero treatment of DiCre×R26R mice. In conclusion, DiCre has indeed the potentiality to be used to establish conditional Cre-deleter mice. An added advantage of this system is that, contrary to other modulatable Cre systems, it offers the possibility of obtaining regulated recombination in a combinatorial manner, i.e. induce recombination at any desired time-point specifically in cells characterized by the simultaneous expression of two different promoters.
Rapamycin (Rap), a small molecule inhibitor of mTOR, is an immunosuppressant and several Rap analogs are cancer chemotherapeutics. Further pharmacological development will be significantly facilitated if in vivo reporter models are available to enable monitoring of molecular-specific pharmacodynamic actions of Rap and its analogs. Herein we present the use of a Gal4→Fluc reporter mouse for the study of Rap-induced mTOR/FKBP12 protein-protein interactions (PPIs) in vivo using a mouse two-hybrid (M2H) transactivation strategy, a derivative of the yeast two-hybrid (Y2H) system applied to live mice. Upon treatment with Rap, a bipartite transactivator was reconstituted and transcription of a genomic firefly luciferase (Fluc) reporter was activated in a concentration-dependent (Kd = 2.3 nM) and FK506-competitive (Ki = 17.1 nM) manner in cellulo as well as in a temporal and specific manner in vivo. In particular, following a single dose of Rap (4.5 mg/kg, i.p.), peak Rap-induced PPIs were observed in the liver at 24 h post treatment with photon flux signals 600-fold over baseline, which correlated temporally with suppression of p70S6K activity, a downstream effector of mTOR. The Gal4→Fluc reporter mouse provides an intact physiological system to interrogate PPIs and molecular-specific pharmacodynamics during drug discovery and lead characterization. Imaging protein interactions and functional proteomics in whole animals in vivo may serve as a basic tool for screening and mechanism-based analysis of small molecules targeting specific PPIs in human diseases.
Improved techniques to non-invasively image protein-protein interactions (PPIs) are essential. We molecularly engineered a positron emission tomography (PET)-based split reporter (herpes simplex virus type 1 thymidine kinase [TK]), split between Thr265 and Ala266, and used this in a protein-fragment complementation assay (PCA) to quantitatively measure PPIs in mammalian cells and to microPET image them in living mice. An introduced point mutation (V119C) significantly enhanced TK complementation in PCAs based on rapamycin modulation of FRB (FKBP12-rapamycin-binding domain) and FKBP12 (FK506 binding protein), on interaction of hypoxia-inducible factor-1α and the von Hippel-Lindau tumor suppressor, and in an estrogen receptor intramolecular protein folding assay. Applications of this novel split TK are potentially far-reaching, including for example considerably more accurate monitoring of immune and stem cell therapies, allowing unprecedented fully quantitative and tomographic PET localization of PPIs in pre-clinical small and large animal models of disease.
Bimolecular fluorescence complementation (BiFC) analysis enables visualization of the subcellular locations of protein interactions in living cells. We investigated the temporal resolution and the quantitative accuracy of BiFC analysis using fragments of different fluorescent proteins. We determined the kinetics of BiFC complex formation in response to the rapamycin-inducible interaction between the FK506 binding protein (FKBP) and the FKBP-rapamycin binding domain (FRB). Fragments of YFP fused to FKBP and FRB produced detectable BiFC complex fluorescence 10 minutes after rapamycin addition and a ten-fold increase in the mean fluorescence intensity in 8 hours. The N-terminal fragment of the Venus fluorescent protein fused to FKBP produced constitutive BiFC complexes with several C-terminal fragments fused to FRB. A chimeric N-terminal fragment containing residues from Venus and YFP produced either constitutive or inducible BiFC complexes depending on the temperature at which the cells were cultured. The concentrations of inducers required for half-maximal induction of BiFC complex formation by all fluorescent protein fragments tested were consistent with the affinities of the inducers for unmodified FKBP and FRB. Treatment of the FK506 inhibitor of FKBP-FRB interaction prevented the formation of BiFC complexes by FKBP and FRB fusions, but did not disrupt existing BiFC complexes. Proteins synthesized prior to rapamycin addition formed BiFC complexes with the same efficiency as newly synthesized proteins. Inhibitors of protein synthesis attenuated BiFC complex formation independent of their effects on fusion protein synthesis. The kinetics at which they inhibited BiFC complex formation suggest that they prevented association of the fluorescent protein fragments, but not the slow maturation of BiFC complex fluorescence. Agents that induce the unfolded protein response also reduced formation of BiFC complexes. The effects of these agents were suppressed by cellular adaptation to protein folding stress. In summary, BiFC analysis enables detection of protein interactions within minutes after complex formation in living cells, but does not allow detection of complex dissociation. Conditional BiFC complex formation depends on the folding efficiencies of fluorescent protein fragments and can be affected by the cellular protein folding environment.
Visualization of protein interactions; bimolecular fluorescence complementation (BiFC); fluorescence microscopy and flow cytometry; protein folding; FKBP-FRB association
Candida albicans and Cryptococcus neoformans cause both superficial and disseminated infections in humans. Current antifungal therapies for deep-seated infections are limited to amphotericin B, flucytosine, and azoles. A limitation is that commonly used azoles are fungistatic in vitro and in vivo. Our studies address the mechanisms of antifungal activity of the immunosuppressive drug rapamycin (sirolimus) and its analogs with decreased immunosuppressive activity. C. albicans rbp1/rbp1 mutant strains lacking a homolog of the FK506-rapamycin target protein FKBP12 were found to be viable and resistant to rapamycin and its analogs. Rapamycin and analogs promoted FKBP12 binding to the wild-type Tor1 kinase but not to a rapamycin-resistant Tor1 mutant kinase (S1972R). FKBP12 and TOR mutations conferred resistance to rapamycin and its analogs in C. albicans, C. neoformans, and Saccharomyces cerevisiae. Our findings demonstrate the antifungal activity of rapamycin and rapamycin analogs is mediated via conserved complexes with FKBP12 and Tor kinase homologs in divergent yeasts. Taken together with our observations that rapamycin and its analogs are fungicidal and that spontaneous drug resistance occurs at a low rate, these mechanistic findings support continued investigation of rapamycin analogs as novel antifungal agents.
FK506 binding protein 51 (FKBP51) is an immunophilin physiologically expressed in lymphocytes. Very recently, aberrant expression of this protein was found in melanoma; FKBP51 expression correlates with melanoma aggressiveness and is maximal in metastatic lesions. FKBP51 promotes NF-κB activation and is involved in the resistance to genotoxic agents, including anthracyclines and ionizing radiation. FKBP51 is a cochaperone with peptidyl-prolyl isomerase activity that regulates several biological processes through protein-protein interaction. There is increasing evidence that FKBP51 hyperexpression is associated with cancer and this protein has a relevant role in sustaining cell growth, malignancy, and resistance to therapy. There is also evidence that FKBP ligands are potent anticancer agents, in addition to their immunosuppressant activity. In particular, rapamycin and its analogs have shown antitumor activity across a variety of human cancers in clinical trials. Although, classically, rapamycin actions are ascribed to inhibition of mTOR, recent studies indicate FKBP51 is also an important molecular determinant of the drug’s anticancer activity. The aim of this article is to review the functions of FKBP51, especially in view of the recent findings that this protein is a potential oncogene when deregulated and a candidate target for signaling therapies against cancer.
FKBP51; cancer; apoptosis; NF-κB; rapamycin.
A light-activatable bacteriophage T7 RNA polymerase (T7RNAP) has been generated through the site-specific introduction of a photocaged tyrosine residue at the crucial position Tyr639 within the active site of the enzyme. The photocaged tyrosine disrupts polymerase activity by blocking the incoming nucleotide from reaching the active site of the enzyme. However, a brief irradiation with nonphototoxic UV light of 365 nm removes the ortho-nitrobenzyl caging group from Tyr639 and restores the RNA polymerase activity of T7RNAP. The complete orthogonality of T7RNAP to all endogenous RNA polymerases in pro- and eukaryotic systems allowed for the photochemical activation of gene expression in bacterial and mammalian cells. Specifically, E. coli cells were engineered to produce photocaged T7RNAP in the presence of a GFP reporter gene under the control of a T7 promoter. UV irradiation of these cells led to the spatiotemporal activation of GFP expression. In an analogous fashion, caged T7RNAP was transfected into human embryonic kidney (HEK293T) cells. Irradiation with UV light led to the activation of T7RNAP, thereby inducing RNA polymerization and expression of a luciferase reporter gene in tissue culture. The ability to achieve spatiotemporal regulation of orthogonal RNA synthesis enables the precise dissection and manipulation of a wide range of cellular events, including gene function.
amino acids; caged proteins; light activation; polymerases; RNA
The FK506-binding protein 51 (FKBP51) is an Hsp90-associated co-chaperone which regulates steroid receptors and kinases. In pancreatic cancer cell lines, FKBP51 was shown to recruit the phosphatase PHLPP to facilitate dephosphorylation of the kinase Akt, which was associated with reduced chemoresistance. Here we show that in addition to FKBP51 several other members of the FKBP family bind directly to Akt. FKBP51 can also form complexes with other AGC kinases and mapping studies revealed that FKBP51 interacts with Akt via multiple domains independent of their activation or phosphorylation status. The FKBP51-Akt1 interaction was not affected by FK506 analogs or Akt active site inhibitors, but was abolished by the allosteric Akt inhibitor VIII. None of the FKBP51 inhibitors affected AktS473 phosphorylation or downstream targets of Akt. In summary, we show that FKBP51 binds to Akt directly as well as via Hsp90. The FKBP51-Akt interaction is sensitive to the conformation of Akt1, but does not depend on the FK506-binding pocket of FKBP51. Therefore, FKBP inhibitors are unlikely to inhibit the Akt-FKBP-PHLPP network.
Studies of cellular and tissue dynamics benefit greatly from tools that can control protein activity with specificity and precise timing in living systems. We describe here a new approach to confer allosteric regulation specifically on the catalytic activity of kinases. A highly conserved portion of the kinase catalytic domain is modified with a small protein insert that inactivates catalytic activity, but does not affect other protein interactions. Catalytic activity is restored by addition of rapamycin or non-immunosuppresive analogs (Fig. 1A). We demonstrate the approach by specifically activating focal adhesion kinase (FAK) within minutes in living cells, thereby demonstrating a novel role for FAK in regulation of membrane dynamics. Molecular modeling and mutagenesis indicate that the protein insert reduces activity by increasing the flexibility of the catalytic domain. Drug binding restores activity by increasing rigidity. Successful regulation of Src and p38 suggest that modification of this highly conserved site will be applicable to other kinases.
The four members of the ErbB family of receptor tyrosine kinases are involved in a complex array of combinatorial interactions involving homo- and heterodimers. Since most cell types express more than one member of the ErbB family, it is difficult to distinguish the biological activities of different homo- and heterodimers. Here we describe a method for inducing homo- or heterodimerization of ErbB receptors by using synthetic ligands without interference from the endogenous receptors. ErbB receptor chimeras containing synthetic ligand binding domains (FK506-binding protein [FKBP] or FKBP-rapamycin-binding domain [FRB]) were homodimerized with the bivalent FKBP ligand AP1510 and heterodimerized with the bifunctional FKBP-FRB ligand rapamycin. AP1510 treatment induced tyrosine phosphorylation of ErbB1 and ErbB2 homodimers and recruitment of Src homology 2 domain-containing proteins (Shc and Grb2). In addition, ErbB1 and ErbB2 homodimers activated downstream signaling pathways leading to Erk2 and Akt phosphorylation. However, only ErbB1 homodimers were internalized upon AP1510 stimulation, and only ErbB1 homodimers were able to associate with and induce phosphorylation of c-Cbl. Cells expressing AP1510-induced ErbB1 homodimers were able to associate with and induce phosphorylation of c-Cbl. Cells expressing AP1510-induced ErbB1 homodimers were able to form foci; however, cells expressing ErbB2 homodimers displayed a five- to sevenfold higher focus-forming ability. Using rapamycin-inducible heterodimerization we show that c-Cbl is unable to associate with ErbB1 in a ErbB1-ErbB2 heterodimer most likely because ErbB2 is unable to phosphorylate the c-Cbl binding site on ErbB1. Thus, we demonstrate that ErbB1 and ErbB2 homodimers differ in their abilities to transform fibroblasts and provide evidence for differential signaling by ErbB homodimers and heterodimers. These observations also validate the use of synthetic ligands to study the signaling and biological specificity of selected ErbB dimers in any cell type.
FK506 binding protein 51 (FKBP51, also called FKBP5) belongs to a family of immunophilins, FK506 binding proteins (FKBPs). FKBP family members are targets for drugs such as rapamycin. Although FKBP51 shares characteristics with other FKBPs, it also has unique features, especially the role in its regulation of important signaling pathways such as the AKT kinase/protein kinase B pathway. In this review, we will focus on the function of FKBP51 as a scaffolding protein in the regulation of AKT activation and, in turn, its role in tumorigenesis and response to chemotherapy.
AKT; AKT phosphorylation; FKBP51; Scaffolding protein; chemoresistance; chemosensitivity
Morpholino oligonucleotides, or morpholinos, have emerged as powerful antisense reagents for evaluating gene function in both in vitro and in vivo contexts. However, the constitutive activity of these reagents limits their utility for applications that require spatiotemporal control, such as tissue specific gene disruptions in embryos. In addition, current indirect methods for spatiotemporal regulation of morpholino activity in vivo may have off-target effects. Here we report a novel and efficient synthetic route for directly incorporating photocaged monomeric building blocks into morpholino oligomers, and demonstrate the utility of these caged morpholinos in the light-activated control of gene function in both cell culture and living embryos. We demonstrate that a caged morpholino targeting enhanced green fluorescent protein (EGFP) disrupts EGFP production only after exposure to UV light in both transfected cells and living zebrafish (Danio rerio) and Xenopus frog embryos. Finally, we show that a caged morpholino targeting chordin, a zebrafish gene that yields a distinct phenotype when functionally disrupted by conventional morpholinos, elicits a chordin phenotype in a UV-dependent manner. Our results suggest that directly photocaged morpholinos are readily synthesized and highly efficacious tools for light-activated spatio-temporal control of gene expression in multiple contexts.
antisense agents; gene expression; light-activation; morpholino; photocaging
Mammalian target of rapamycin (mTOR), a large multidomain protein kinase, regulates cell growth and metabolism in response to environmental signals. The FKBP rapamycin-binding (FRB) domain of mTOR is a validated therapeutic target for the development of immunosuppressant and anticancer drugs but is labile and insoluble. Here we designed a fusion protein between FKBP12 and the FRB domain of mTOR. The fusion protein was successfully expressed in Escherichia coli as a soluble form, and was purified by a simple two-step chromatographic procedure. The fusion protein exhibited increased solubility and stability compared with the isolated FRB domain, and facilitated the analysis of rapamycin and FK506 binding using differential scanning calorimetry (DSC) and solution nuclear magnetic resonance (NMR). DSC enabled the rapid observation of protein–drug interactions at the domain level, while NMR gave insights into the protein–drug interactions at the residue level. The use of the FKBP12–FRB fusion protein combined with DSC and NMR provides a useful tool for the efficient screening of FKBP12-dependent as well as -independent inhibitors of the mTOR FRB domain.
DSC; FRB domain; NMR; fusion protein
The ability to rapidly regulate the functions of specific proteins in living cells is a valuable tool for biological research. Here we describe a novel technique by which the degradation of a specific protein is induced by a small molecule. A protein of interest is fused to a Ligand-Induced Degradation (LID) domain resulting in the expression of a stable and functional fusion protein. The LID domain is comprised of the FK506- and rapamycin-binding protein (FKBP) and a 19-amino acid degron fused to the C-terminus of FKBP. In the absence of the small molecule Shield-1, the degron binds to the FKBP protein and the fusion protein is stable. Shield-1 binds tightly to FKBP thereby displacing the degron and inducing rapid and processive degradation of the LID domain and any fused partner protein. Structure-function studies of the 19-residue peptide showed that a four-amino acid sequence within the peptide is responsible for degradation.
In complex with FKBP12, the immunosuppressant rapamycin binds to and inhibits the yeast TOR1 and TOR2 proteins and the mammalian homologue mTOR/FRAP/RAFT1. The TOR proteins promote cell cycle progression in yeast and human cells by regulating translation and polarization of the actin cytoskeleton. A C-terminal domain of the TOR proteins shares identity with protein and lipid kinases, but only one substrate (PHAS-I), and no regulators of the TOR-signaling cascade have been identified. We report here that yeast TOR1 has an intrinsic protein kinase activity capable of phosphorylating PHAS-1, and this activity is abolished by an active site mutation and inhibited by FKBP12-rapamycin or wortmannin. We find that an intact TOR1 kinase domain is essential for TOR1 functions in yeast. Overexpression of a TOR1 kinase-inactive mutant, or of a central region of the TOR proteins distinct from the FRB and kinase domains, was toxic in yeast, and overexpression of wild-type TOR1 suppressed this toxic effect. Expression of the TOR-toxic domain leads to a G1 cell cycle arrest, consistent with an inhibition of TOR function in translation. Overexpression of the PLC1 gene, which encodes the yeast phospholipase C homologue, suppressed growth inhibition by the TOR-toxic domains. In conclusion, our findings identify a toxic effector domain of the TOR proteins that may interact with substrates or regulators of the TOR kinase cascade and that shares sequence identity with other PIK family members, including ATR, Rad3, Mei-41, and ATM.
FK-506-binding proteins (FKBPs), which in T cells are supposed to mediate the immunosuppressive effects of the compounds FK-506 and rapamycin, have been isolated from Streptomyces chrysomallus, S. hygroscopicus subsp. ascomyceticus, and S. hygroscopicus. The latter two strains are producers of ascomycin (the ethyl analog of FK-506) and rapamycin, respectively. Like the 12-kDa FKBP in eukaryotic organisms such as humans, bovines, and Saccharomyces cerevisiae, or the FKBPs from gram-positive streptomycetes are peptidyl-prolyl-cis-trans isomerases. Inhibition studies using FK-506, rapamycin, or ascomycin, revealed inhibition of the peptidyl-prolyl cis-trans isomerase activity of the proteins at the nanomolar level, which is in the same range as with eukaryotic FKBPs. The M(r)s of the various FKBPs were 13,500 to 15,000, and they had the same pI of approximately 4.5. The N-terminal sequences of the three FKBPs were nearly identical in the first 20 amino acids. The amino acid sequence deduced from the gene sequence of S. chrysomallus gave a polypeptide of 124 amino acids. The homologies to FKBPs from humans, S. cerevisiae, and Neurospora crassa were 38, 39, and 50% identity in relevant positions, respectively. Significant homology of 38% was also seen with the C-terminal halves of bacterial protein surface antigens like the Mip protein of Legionella pneumophila and the 27-kDa Mip-like protein of Chlamydia trachomatis. In addition, two more open reading frames in Pseudomonas aeruginosa and Neisseria meningitidis of unknown function show regions of homology to the S. chrysomallus FKBP. In contrast to fungi, streptomycetes are resistant to macrolactones. Ascomycin-producing S. hygroscopicus subsp. ascomyceticus excretes the compound almost quantitatively into medium, which indicates that the organism has an efficient self-protection mechanism against its own secondary metabolite.
Rapamycin is a macrolide antifungal agent that exhibits potent immunosuppressive properties. In Saccharomyces cerevisiae, rapamycin sensitivity is mediated by a specific cytoplasmic receptor which is a homolog of human FKBP12 (hFKBP12). Deletion of the gene for yeast FKBP12 (RBP1) results in recessive drug resistance, and expression of hFKBP12 restores rapamycin sensitivity. These data support the idea that FKBP12 and rapamycin form a toxic complex that corrupts the function of other cellular proteins. To identify such proteins, we isolated dominant rapamycin-resistant mutants both in wild-type haploid and diploid cells and in haploid rbp1::URA3 cells engineered to express hFKBP12. Genetic analysis indicated that the dominant mutations are nonallelic to mutations in RBP1 and define two genes, designated DRR1 and DRR2 (for dominant rapamycin resistance). Mutant copies of DRR1 and DRR2 were cloned from genomic YCp50 libraries by their ability to confer drug resistance in wild-type cells. DNA sequence analysis of a mutant drr1 allele revealed a long open reading frame predicting a novel 2470-amino-acid protein with several motifs suggesting an involvement in intracellular signal transduction, including a leucine zipper near the N terminus, two putative DNA-binding sequences, and a domain that exhibits significant sequence similarity to the 110-kDa catalytic subunit of both yeast (VPS34) and bovine phosphatidylinositol 3-kinases. Genomic disruption of DRR1 in a mutant haploid strain restored drug sensitivity and demonstrated that the gene encodes a nonessential function. DNA sequence comparison of seven independent drr1dom alleles identified single base pair substitutions in the same codon within the phosphatidylinositol 3-kinase domain, resulting in a change of Ser-1972 to Arg or Asn. We conclude either that DRR1 (alone or in combination with DRR2) acts as a target of FKBP12-rapamycin complexes or that a missense mutation in DRR1 allows it to compensate for the function of the normal drug target.
The eukaryotic TOR pathway controls translation, growth and the cell cycle in response to environmental signals such as nutrients or growth-stimulating factors. The TOR protein kinase can be inactivated by the antibiotic rapamycin following the formation of a ternary complex between TOR, rapamycin and FKBP12 proteins. The TOR protein is also found in higher plants despite the fact that they are rapamycin insensitive. Previous findings using the yeast two hybrid system suggest that the FKBP12 plant homolog is unable to form a complex with rapamycin and TOR, while the FRB domain of plant TOR is still able to bind to heterologous FKBP12 in the presence of rapamycin. The resistance to rapamycin is therefore limiting the molecular dissection of the TOR pathway in higher plants.
Here we show that none of the FKBPs from the model plant Arabidopsis (AtFKBPs) is able to form a ternary complex with the FRB domain of AtTOR in the presence of rapamycin in a two hybrid system. An antibody has been raised against the AtTOR protein and binding of recombinant yeast ScFKBP12 to native Arabidopsis TOR in the presence of rapamycin was demonstrated in pull-down experiments. Transgenic lines expressing ScFKBP12 were produced and were found to display a rapamycin-dependent reduction of the primary root growth and a lowered accumulation of high molecular weight polysomes.
These results further strengthen the idea that plant resistance to rapamycin evolved as a consequence of mutations in plant FKBP proteins. The production of rapamycin-sensitive plants through the expression of the ScFKBP12 protein illustrates the conservation of the TOR pathway in eukaryotes. Since AtTOR null mutants were found to be embryo lethal , transgenic ScFKBP12 plants will provide an useful tool for the post-embryonic study of plant TOR functions. This work also establish for the first time a link between TOR activity and translation in plant cells
The mammalian target of rapamycin protein (mTOR) is an evolutionarily conserved kinase that regulates protein synthesis, cell cycle progression and proliferation in response to various environmental cues. As a critical downstream mediator of PI3K signaling, mTOR is important for lymphocyte development and function of mature T and B-cells. Most studies of mTOR in immune responses have relied on the use of pharmacological inhibitors, such as rapamycin. Rapamycin-FKBP12 complex exerts its immunosuppressive and anti-proliferative effect by binding outside the kinase domain of mTOR, and subsequently inhibiting downstream mTOR signaling.
To determine the requirement for mTOR kinase activity in the immune system function, we generated knock-in mice carrying a mutation (D2338) in the catalytic domain of mTOR. While homozygous mTOR kd/kd embryos died before embryonic day 6.5, heterozygous mTOR+/kd mice appeared entirely normal and are fertile. mTOR +/kd mice exhibited normal T and B cell development and unaltered proliferative responses of splenocytes to IL-2 and TCR/CD28. In addition, heterozygousity for the mTOR kinase-dead allele did not sensitize T cells to rapamycin in a CD3-mediated proliferation assay. Unexpectedly, mTOR kinase activity towards its substrate 4E-BP1 was not decreased in hearts and livers from heterozygous animals.
Altogether, our findings indicate that mTOR kinase activity is indispensable for the early development of mouse embryos. Moreover, a single wild type mTOR allele is sufficient to maintain normal postnatal growth and lymphocyte development and proliferation.
Inhibitors of the phosphatidylinositol 3-kinase (PI3 kinase)–FKBP-rapamycin-associated protein (FRAP) pathway, such as rapamycin and wortmannin, induce dephosphorylation and activation of the suppressor of cap-dependent translation, 4E-BP1. Encephalomyocarditis virus (EMCV) infection leads to activation of 4E-BP1 at the time of host translation shutoff. Consistent with these data, rapamycin mildly enhances the synthesis of viral proteins and the shutoff of host cell protein synthesis after EMCV infection. In this study, two defective EMCV strains were generated by deleting portions of the 2A coding region of an infectious cDNA clone. These deletions dramatically decreased the efficiency of viral protein synthesis and abolished the virus-induced shutoff of host translation after infection of BHK-21 cells. Both translation and processing of the P1-2A capsid precursor polypeptide are impaired by the deletions in 2A. The translation and yield of mutant viruses were increased significantly by the presence of rapamycin and wortmannin during infection. Thus, inhibition of the PI3 kinase-FRAP signaling pathway partly complements mutations in 2A protein and reverses a slow-virus phenotype.
Rapamycin is a macrolide antifungal agent with structural similarity to FK506. It exhibits potent immunosuppressive properties analogous to those of both FK506 and cyclosporin A (CsA). Unlike FK506 and CsA, however, rapamycin does not inhibit the transcription of early T-cell activation genes, including interleukin-2, but instead appears to block downstream events leading to T-cell activation. FK506 and CsA receptor proteins (FKBP and cyclophilin, respectively) have been identified and shown to be distinct members of a class of enzymes that possess peptidyl-prolyl cis-trans isomerase (PPIase) activity. Despite the apparent differences in their mode of action, rapamycin and FK506 act as reciprocal antagonists in vivo and compete for binding to FKBP. As a means of rapidly identifying a target protein for rapamycin in vivo, we selected and genetically characterized rapamycin-resistant mutants of Saccharomyces cerevisiae and isolated a yeast genomic fragment that confers drug sensitivity. We demonstrate that the resonse to rapamycin in yeast cells is mediated by a gene encoding a 114-amino-acid, approximately 13-kDa protein which has a high degree of sequence homology with human FKBP; we designated this gene RBP1 (for rapamycin-binding protein). The RBP1 protein (RBP) was expressed in Escherichia coli, purified to homogeneity, and shown to catalyze peptidyl-prolyl isomerization of a synthetic peptide substrate. PPIase activity was completely inhibited by rapamycin and FK506 but not by CsA, indicating that both macrolides bind to the recombinant protein. Expression of human FKBP in rapamycin-resistant mutants restored rapamycin sensitivity, indicating a functional equivalence between the yeast and human enzymes.
FK-506 inhibits Ca(2+)-dependent transcription of lymphokine genes in T cells, and thereby acts as a powerful immunosuppressant. However, its potential therapeutic applications may be seriously limited by several side effects, including nephrotoxicity and neurotoxicity. At present, it is unclear whether these immunosuppressive and toxic effects result from interference with related biochemical processes. FK-506 is known to interact with FK-binding protein-12 (FKBP-12), an abundant cytosolic protein with cis-trans peptidyl-prolyl isomerase activity (PPIase) activity. Because rapamycin (RAP) similarly binds to FKBP-12, although it acts in a manner different from FK-506, by inhibiting T cell responses to lymphokines, such an interaction with FKBP-12 is not sufficient to mediate immunosuppression. Recently, it was found that the complex of FKBP-12 with FK-506, but not with RAP, inhibits the phosphatase activity of calcineurin. Here, we used L-685,818, the C18- hydroxy, C21-ethyl derivative of FK-506, to explore further the role of FKBP-12 in the immunosuppressive and toxic actions of FK-506. Although L-685,818 bound with high affinity to FKBP-12 and inhibited its PPIase activity, it did not suppress T cell activation, and, when complexed with FKBP-12, did not affect calcineurin phosphatase activity. However, L-685,818 was a potent antagonist of the immunosuppressive activity of both FK-506 and RAP. Moreover, L-685,818 did not induce any toxicity in dogs and rats or in a mouse model of acute FK-506 nephrotoxicity, but it blocked the effect of FK-506 in this model. Therefore, FK-506 toxicity involves the disruption of biochemical mechanisms related to those implicated in T cell activation. Like immunosuppression, this toxicity is not due to the inhibition of the PPIase activity of FKBP- 12, but may be linked to the inhibition of the phosphatase activity of calcineurin by the drug FKBP-12 complex.
The potent antiproliferative activity of the macrolide antibiotic rapamycin is known to involve binding of the drug to its cytosolic receptor, FKBP12, and subsequent interaction with targets of rapamycin, resulting in inhibition of p70 S6 kinase (p70S6K). However, the downstream events that lead to inhibition of cell cycle progression remain to be elucidated. The antiproliferative effects of rapamycin are associated with prevention of mitogen-induced downregulation of the cyclin-dependent kinase inhibitor p27Kip1, suggesting that the latter may play an important role in the growth pathway targeted by rapamycin. Murine BC3H1 cells, selected for resistance to growth inhibition by rapamycin, exhibited an intact p70S6K pathway but had abnormally low p27 levels that were no longer responsive to mitogens or rapamycin. Fibroblasts and T lymphocytes from mice with a targeted disruption of the p27Kip1 gene had impaired growth-inhibitory responses to rapamycin. These results suggest that the ability to regulate p27Kip1 levels is important for rapamycin to exert its antiproliferative effects.